1. School of Electronic Science and Applied Physics; Hefei University of Technology; Hefei 230009 P. R. China
2. Center of Super-Diamond and Advanced Films (COSDAF); City University of Hong Kong; Hong Kong SAR P. R. China
3. Department of Physics and Materials Science; City University of Hong Kong; Hong Kong SAR P. R. China
4. School of Energy and Environment; City University of Hong Kong; Hong Kong SAR P. R. China